The Effect of Feedback Quantization on the Throughput of a Multiuser Diversity Scheme

The impact of the quantization of SNR measurements on the throughput of a multiuser diversity scheme for constant-rate transmission is investigated under a block-Rayleigh fading assumption. In the downlink, each user measures its SNR, quantizes it, and feeds it back to the transmitter, which transmits a packet to the user with the highest quantized SNR. In the case of several users having the same quantized SNR, one of them is selected at random. It is concluded that using only a few quantization levels can yield a throughput that is only slightly less than the throughput obtained by using unquantized feedback

Throughput analysis of three multiuser diversity schemes

The throughput of three slot allocation schemes for the downlink of a wireless TDMA system are investigated under the assumption of block-Rayleigh fading. The schemes use the instantaneous SNR at the users in order to take advantage of the independent fading of the channels. In the first scheme, which has been proposed earlier (R. Knopp et al., 1995), the base station transmits to the user that experiences the largest SNR in every slot. In the second scheme, which is a variant of an existing scheme (P. Viswanath et al., June 2002), the user with highest ratio between instantaneous SNR and mean SNR is transmitted to. Lastly, we propose a scheme that compares the SNR at a user to a threshold in order determine if transmission to that user shall occur. If the SNR is below the threshold, transmission to another user is attempted. The results indicate that considerable throughput gains can be achieved even with limited SNR feedback, although there exists a trade-off between throughput and dela

Throughput analysis of strongly interfering slow frequency-hopping wireless networks

We derive an approximation for the throughput of strongly interfering frequency-hopping wireless networks, where packet collisions always result in lost data. A system is defined to consist of a certain number of radio networks, each with an arbitrary number of communicating units, coordinated to communicate without interference. Using the approximation, we estimate upper and lower bounds on system throughput, as well as the number of networks which gives maximum system throughpu

Aspects on Interference and Diversity in Wireless Networks

This thesis deals with two topics in wireless communications: interference between frequency-hopping (FH) networks, and multiuser diversity. The work on FH treats a system where FH is used to provide simultaneous access to the channel for multiple uncoordinated wireless networks. This is achieved by performing frequency hops on a packet-by-packet basis, and letting the sequence of hopping frequencies be pseudo random. Furthermore, the networks in the considered system are allowed to use several packet types, which can be of different lengths. A throughput analysis of this system is performed, and this analysis is based mainly on two assumptions. Firstly, collisions, i.e., overlaps in time and frequency between packets, are assumed to always result in a total loss of the data in the colliding packets. Secondly, it is assumed that networks are asynchronous. Both exact and approximate expressions for the throughput are derived, and the dependence of the throughput on the number of networks and the available packet types is investigated for two existing wireless systems, namely Bluetooth and IEEE 802.11 FHSS. Furthermore, the accuracy of the collision assumption as a function of the distance between networks is determined for the Bluetooth system. The second topic of this thesis, multiuser diversity, is a scheme where a base station collects channel state information from a number of users and then transmits to the user with the instantaneously best channel. Although this scheme promises large gains, there are a number of practical aspects that reduce this gain. In this thesis two of these aspects are considered. Firstly, the impact of quantizing the feedback of the users' signal-to-noise ratio measurements is investigated for both variable- and constant-rate transmission. It is shown that even for a coarse quantization, the multiuser diversity gain is significant and good performance is achieved. Secondly, the impact of feedback delay is analyzed while also taking into account the additional diversity, referred to as link diversity, that Rake reception or transmit diversity (using space--time block codes) can provide. Expressions for the error rates and spectral efficiencies for uncoded constant- and variable-rate transmission are derived, and the dependence on the number of users, the feedback delay, and the link diversity order is investigated. These expressions also apply to Rake reception combined with transmit or receive antenna selection

Throughput of IEEE 802.11 FHSS networks in the presence of strongly interfering Bluetooth networks

The impact of interference from Bluetooth networks on the throughput of IEEE 802.11 FHSS networks is investigated. This is done by deriving an analytical approximation of the throughput of slow frequency-hopping systems. The derivation in itself provides valuable insights into the mechanisms of interference between systems employing the frequency-hopping technique. In deriving the approximation, it is assumed that packet collisions result in total loss of all information contained in the packets involved in the collisions, regardless of the distance between the networks. The results indicate that the Bluetooth networks may have a negative effect on the throughput of an IEEE 802.11 network using long packet type

Analysis of strongly interfering slow frequency-hopping systems

In this report slow frequency-hopping systems are analyzed with respect to the probability of successful transmission of packets, and with respect to the throughput. It is assumed that collisions result in a loss of all information contained in the colliding packets, irrespective of the distance between the units of the system. Units transmit packets of different types with a certain probability, and the packet types can be of different lengths. Two main modes of transmission are treated: synchronous and asynchronous transmission. In the former case, units can only begin transmission of packets at multiples of a common slot time. In the asynchronous case, no such restriction exists. Under the given assumptions, we arrive at analytical expressions for the probability of successful transmission of packets and for the throughput. In addition, approximations of the analytical expressions are given. Finally, two examples are given that show how our system model can be mapped onto existing systems. In the first exampl